Steel Selection for Offshore Structures

Abstract

Failure analysis of salvaged offshore structures has shown that inferior quality steel, poor quality fabrication, and possible overload contributed to structural failure. Evaluation of structures has indicated that there is a owed for improved quality steels in offshore use. Introduction Analysis of salvaged offshore drilling and production structures has shown that inferior quality steel, poor quality fabrication, and overload of the structures were contributing factors for some early structural failures. New specifications for steel, welding, and inspection are being used that will prevent recurrence of the recognized problems. Ordinary structural-steel specifications do not insure adequate mechanical and metallurgical properties known to be required for critical areas of offshore structures. In general, the static load on offshore platform jacket results in very low stresses in the members. Under storm conditions, the stresses are significantly higher and may be both tensile and compressive, with the compressive load usually controlling the design. The most critical area of concern is near the welded joints where two or more structural members intersect (Fig. 1 detail). Areas of high stress concentration occur in each joint. Under extreme conditions, local plastic straining can be expected at these locations. plastic straining can be expected at these locations. Selection of a satisfactory steel requires a practical combination of mechanical and metallurgical properties and economics. Consideration must be given to properties and economics. Consideration must be given to static strength, high- and low-cycle fatigue strength, notch toughness, freedom from laminations, susceptibility to lamellar tearing, weldability, availability, and cost. A large portion of all offshore platforms are fabricated from steel pipe that is fabricated from steel plate. The cost of the steel plate and pipe fabrication plate. The cost of the steel plate and pipe fabrication is significant to the over-all cost of platform construction. An API work group, under the jurisdiction of the API Committee for Standardization of Offshore Structures, bas developed a new specification (API Specification 2H) for carbon-manganese steel plate for offshore platform tubular joints. The Continental Oil Co. specification and the new API specification provide similar steel properties for similar provide similar steel properties for similar applications. Invoice and quotation price comparisons were made between steel purchased to an earlier Conoco specification (similar to Table 1) and alternate materials. Using the Conoco specification resulted in the same or lower costs when compared with the cost of adding supplementary restrictions or requirements to ordinary structural steels or to comparable pressure-vessel steels. pressure-vessel steels. The offshore oil industry is faced with the challenge of building even larger offshore structures, in deeper water, for more severe environments, with improved service life and reliability. The selection of adequate quality steel at reasonable cost is mandatory if we are to meet this challenge. Analysis of Structural Failures Comprehensive failure-analysis studies have been made on a number of failures from offshore platforms. JPT P. 274